The present invention relates to a battery charging circuit, and more particularly, to a battery charging circuit and charging method suitable for charging a battery that supplies power to a motor drive system having a motor such as a vehicle-driving motor and a three-phase inverter for driving the motor, for example, a motor drive system in an electric vehicle or a hybrid vehicle.
A large-sized battery mounted as a power source on a vehicle such as an electric vehicle needs to be charged when the amount of charge is less than or equal to a predetermined amount.
Conventionally, a liquid type lead-acid battery is charged via a quasi-constant voltage transformer and a rectifier circuit. In this charging method, the charging current is greater when the battery voltage is low. As the battery voltage increases, the charging current is reduced.
In the charging method using a quasi-constant voltage transformer, the charging performance depends on the input voltage and the state of the battery. Thus, insufficient charging or overcharging can occur. In a method for performing constant current charging, for example, the constant current-constant voltage-constant current (CC-CV-CC) method, the charging performance does not depend on the input voltage or the state of the battery. However, the method requires a great number of components for the charger, and is therefore costly.
Conventionally, a charger for electric vehicles has been proposed that uses coils in a three-phase motor and a three-phase inverter mounted on an electric vehicle. For example, Japanese Laid-Open Patent Publication No. 8-126122 discloses a charger in which a connector is provided in a part of a line that connects coils of the motor and switching elements of the inverter. When the motor is driven by the battery, the connector is connected. When the battery is charged by an outside alternating-current power source, the connector is disconnected. The publication also discloses a charger without such a connector. In this charger, the positive terminal of a rectifier, which is connected to the outside alternating-current power source, is connected to the neutral point of the coils of the three-phase motor that are connected via a star connection.
The charger of the above publication requires that a connector be provided at a part of the line connecting a motor coil and a switching element of the inverter, and that a rectifier, which is connected to an outside alternating-current power source, be connected to the neutral point of motor coils in a star connection. Therefore, when such a charger (charging circuit) is formed by using a conventional circuit that controls a three-phase motor via a three-phase inverter, not only do components have to be added, but also circuit wiring needs to be modified. This complicates the configuration of the charger.